PROJECT SUMMARY Aberrant expression of a family of basic helix-loop-helix (bHLH) transcription factors and two LIM domain-containing transcriptional adaptors, LIM-only protein 1 (LMO1) and 2 (LMO2), constitute the most frequent gain-of-function abnormalities in T-cell acute lymphoblastic leukemia (T-ALL). This can result from either chromosomal translocation or dysregulated mono- or bi-allelic expression. The protein products of these genes contribute to a common DNA-binding complex that also contains the LIM domain-binding protein Ldb1 and two Ldb1-interacting proteins, single stranded DNA-binding protein-2 (SSBP2) and -3 (SSBP3). This or a related complex mediates the oncogenic actions of these nuclear proteins by activating and repressing the transcription of specific sets of target genes. Although the importance of both bHLH and LMO proteins to the development of T-ALL is well established, how this multi-protein complex contributes to the phenotype of the malignant lymphoblast and whether interactions between its component proteins can be targeted for therapeutic purpose are not known. We have determined that the SSBPs protect Ldb1 and LMO proteins from ubiquitylation and proteasomal destruction and, through a separate mechanism, promote Ldb1 dimerization. We hypothesize that reducing Ldb1 expression or inhibiting its interaction with itself, SSBPs, or LMO proteins will adversely affect leukemia cell survival, proliferation, and/or function. This wil be tested in three specific aims. In the first aim, the effects of reducing Ldb1 expression by RNA interference will be tested on the viability, number, and invasiveness of leukemia cells derived from two transgenic mouse models of T-ALL. In the second aim, the effects of inhibiting Ldb1 interaction with itself, SSBPs, and LMO proteins will be investigated through use of dominant interfering Ldb1-derived polypeptides corresponding to its N-terminal dimerization domain, central Ldb1/Chip conserved domain, and C- terminal LIM interaction domain, respectively. In the third aim, the effects of reducing or inactivating Ldb1 expression will be tested on leukemia-initiating activity (leukemia stem cell function) in vivo in cell transplantation studies. These studies should provide new insights into Ldb1 function in leukemia cell biology and lead to the development of novel targeted therapies for T-ALL and other malignancies characterized by abnormal expression of bHLH and LMO proteins.